1. Field of the Invention
The present invention relates to a pixel circuit including a light emitting element such as an organic electroluminescence (EL) light emitting element and an active matrix type display device, and a method of manufacturing the pixel circuit.
2. Description of the Related Art
In an image display device, for example, a liquid crystal display device or the like, a large number of pixels are arranged in matrix, and an image is displayed by controlling a light intensity every pixel in correspondence to information on an image to be displayed.
This is also applied to an organic EL display device. However, the organic EL display device is a so-called self-emission type display device having a light emitting element in each of pixel circuits. Thus, the organic EL display device has such advantages that visibility for an image is high, no backlight is required, a response speed is high, and so forth as compared with a liquid crystal display device.
In addition, the organic EL display device is largely different from the liquid crystal display device or the like in that a light emitting element is of a current control type in which a luminance of each of the light emitting elements is controlled with a value of a current caused to flow therethrough, thereby obtaining gradation of coloring.
The organic EL display device can adopt, as its driving system, a simple matrix system and an active matrix system similarly to the case of the liquid crystal display device. However, the former involves such a problem that it is difficult to realize a large and high-definition display device, and so forth although a simple structure can be adopted. In order to solve this problem, an active matrix system is actively developed in which a current caused to flow through a light emitting element provided inside each of pixel circuits is controlled by an active element provided inside each of the pixel circuits, generally, a thin film transistor (TFT).
FIG. 1 is a block diagram showing a structure of a general organic EL display device.
This organic EL display device, as shown in FIG. 1, includes a pixel array portion 2, a horizontal selector (HSEL) 3, a write scanner (WSCN) 4, signal lines (data lines) SGL1 to SGLn, and scanning lines WSL1 to WSLm. Here, in the pixel array portion 2, pixel circuits (PXLC) 2a are disposed in matrix of m×n. The signal lines SGL1 to SGLn are successively selected by the horizontal selector 3, and a data signal corresponding to luminance information is successively supplied to the signal lines SGL1 to SGLn. Also, the scanning lines WSL1 to WSLm are successively selected and driven by the write scanner 4.
It is noted that the horizontal selector 3 and the write scanner 4 may be formed on polycrystalline silicon, or may be formed in the form of MOSICs or the like in the periphery of the pixels.
FIG. 2 is a circuit diagram showing a structural example of the pixel circuit 2a shown in FIG. 1. This circuit diagram, for example, is shown in U.S. Pat. No. 5,684,365 (Patent Document 1), and Japanese Patent Laid-Open No. Hei 8-234683 (Patent Document 2).
The pixel circuit shown in FIG. 2 has the simplest circuit structure among those of a large number of pixel circuits proposed, and is a circuit utilizing a 2 transistor driving system.
The pixel circuit 2a of FIG. 2 includes a p-channel thin film field effect transistor (hereinafter referred to as “a TFT”) 11 and a TFT 12, a capacitor C11, and an organic EL light emitting element (OLED) 13 as a light emitting element. In addition, in FIG. 2, reference symbol SGL designates a signal line, and reference symbol WSL designates a scanning line.
The organic EL light emitting element may be called an organic light emitting diode (OLED) in many cases because it has a rectifying property. For this reason, the light emitting element adopts a symbol of a diode in FIG. 2 and others. However, in the following description, the rectifying property is not necessarily required for the organic light emitting diode.
Referring to FIG. 2, a source of the TFT 11 is connected to a power source potential Vcc, and a cathode of the organic EL light emitting element 13 is connected to a ground potential GND. An operation of the pixel circuit 2a shown in FIG. 2 is as follows.
Step ST1:
When the scanning line WSL is held in a selection state (at a low level in this case), and a write potential Vdata is applied to the signal line SGL, the TFT 12 is turned ON to charge up or discharge the capacitor C11. As a result, a gate potential of the TFT 11 becomes the write potential Vdata.
Step ST2:
When the scanning line WSL is held in a nonselection state (at a high level in this case), the signal line SGL and the TFT 11 are electrically disconnected to each other. However, the gate potential of the TFT 11 is stably held by the capacitor C11.
Step ST3:
A current caused to flow through the TFT 11 and the organic EL light emitting element 13 comes to have a value corresponding to a gate to source voltage of the TFT 11. Thus, the organic EL light emitting element 13 continues to emit a light with the luminance corresponding to this current value.
An operation for selecting the scanning line WSL, thereby transmitting the luminance information given to the data line to the inside of the pixel as in Step ST1 described above will be referred hereinafter as “write”.
As described above, in the pixel circuit 2a shown in FIG. 2, when once the write potential Vdata is written thereto, the organic EL light emitting element 13 continues to emit a light with given luminance until the next rewrite is carried out.
As described above, in the pixel circuit 2a, the value of the current caused to flow through the organic EL light emitting element 13 is controlled by changing the voltage applied to a gate of the TFT 11 as the drive transistor.
At this time, a source of the p-channel drive transistor is connected to the power source potential Vcc, and thus the TFT 11 usually operates in a saturation region. Therefore, the TFT 11 operates as a constant current source having a current value shown in the following expression (1):Ids=½·μ(W/L)Cox(Vgs−|Vth|)2  (1)
Where μ represents a mobility of a carrier, Cox represents a gate capacity per unit area, W represents a gate width, L represents a gate length, Vgs represent a gate to source voltage of the TFT 11, and Vth represents a threshold of the TFT 11.
In the simple matrix type image display device, each of the light emitting elements emits a light only for a selected moment. On the other hand, in the active matrix type image display device, as described above, each of the light emitting elements continues to emit a light even after completion of the write operation. From this reason, the active matrix type image display device is more advantageous, especially, in the large and high-definition display device than the simple matrix type image display device in that a peak luminance and a peak current of the light emitting element are reduced, and so forth.
FIG. 3 is a graphical representation showing deterioration with time of current−voltage (I−V) characteristics of the organic EL light emitting element. Referring to FIG. 3, a curve indicated by a solid line represents the I−V characteristics in a phase of an initial state, and a curve indicated by a broken line represents the I−V characteristics after the deterioration with time.
In general, as shown in FIG. 3, the I−V characteristics of the organic EL light emitting element deteriorate as time elapses.
However, as described above, the constant current continues to be caused to flow through the organic EL light emitting element because the 2 transistor driving system shown in FIG. 2 is the constant current driving system. As a result, even when the I−V characteristics of the organic EL light emitting element deteriorate, an emission luminance of the organic EL light emitting element is free from the deterioration with time.
Now, the pixel circuit 2a of FIG. 2 is constituted by the p-channel TFT. However, if the pixel circuit 2a of FIG. 2 can be constituted by an n-channel TFT, the existing amorphous silicon (s-Si) process can be used in manufacturing a TFT. This leads to that the low-cost promotion for a TFT substrate becomes possible.
Next, a description will now be given with respect to the basic pixel circuit in which the p-channel TFTs are replaced with the n-channel TFT, respectively.
FIG. 4 is a circuit diagram showing a pixel circuit in which the p-channel TFTs in the pixel circuit of FIG. 2 are replaced with the n-channel TFT, respectively.
A pixel circuit 2b of FIG. 4 includes n-channel TFTs 21 and 22, a capacitor C21, and an organic EL light emitting element (OLED) 23 as the light emitting element. In addition, in FIG. 4, reference symbol SGL designates a data line, and reference symbol WSL designates a scanning line.
In this pixel circuit 2b, a drain side of the n-channel TFT 21 as a drive transistor is connected to a power source potential Vcc, and a source thereof is connected to an anode of the organic EL light emitting element 23, thereby forming a source follower circuit.
FIG. 5 is a graphical representation showing an operating point between the n-channel TFT 21 as the drive transistor, and the organic EL light emitting element 23 in an initial state. In FIG. 5, an axis of abscissa represents a drain to source voltage Vds of the n-channel TFT 21, and an axis of ordinate represents a drain to source current Ids of the n-channel TFT 21.
As shown in FIG. 5, a source voltage depends on the operating point between the n-channel TFT 21 as the drive transistor and the organic EL light emitting element 23. Thus, the source voltage takes values different from one another depending on a gate voltage.
Since the n-channel TFT 21 is driven in a saturation region, it causes the current Ids having a current value expressed by the equation form shown in the above expression (1) with respect to the gate to source voltage Vgs corresponding to the source voltage at the operating point.